The present invention relates to cementing techniques used in civil engineering, in the building industry and more particularly in drilling oil wells or the like. More precisely, the invention relates to very low density cementing compositions.
There are many applications for which a light cement would be appropriate. In the civil engineering and building industries, a low density cement could enable less bulky understructures to be produced which would not need to be reinforced to support the weight of the cement. However, the properties of currently available light cements are generally poorer, in particular as regards compressive strength, and the permeability is too high and thus they can only rarely be substituted for ordinary cement, particularly in respect of guaranteeing the work for the desired service life.
In oil wells, choosing a slurry density depends on a number of criteria. The main purpose of the cement placed between the casing and the well wall is to isolate the different geological layers which are traversed and to strengthen the casing mechanically. The cement also protects the steel of the casing from corrosion, by passivating it. In order to avoid any risk of a blow-out, the density of the cement must be adjusted so that the pressure at the bottom of the well is at least equal to the pore pressure in the geological formations traversed. Clearly, the longer the column, the less dense the cement slurry needs to be.
In addition to this lower limit, there is an upper limit on the density. The pressure exerted on the rock (due to the hydrostatic pressure generated by the cement column and to the pressure drop associated with the circulation of fluid during pumping) must be lower than the pressure which that rock can tolerate without fracturing. That pressure increases with the length of the cement column. In general, the length of the cemented section will thus be limited by the density of the cement slurry which could be used.
Since the cement must have a minimum density in order to have acceptable mechanical properties, the length of the cemented section is very often limited by the fracturing pressure if it is not limited for other reasons such as pressure inversions between geological layers. Each new section must be drilled with a smaller diameter than the preceding section to enable the drilling tool and the casing to be lowered through sections already provided with a casing; a section which was too narrow to accommodate the completion tools would be useless. For this reason, if the number of sections is high, drilling must be commenced using large diameter sections at the top of the well, resulting in high extra costs due to an increase in the volume of rock to be drilled and to the greater weight of the casing sections because of their larger diameter. It is, of course, known to cement a section in a plurality of steps to prevent the well from contracting. That technique involves high supplemental costs and the equipment required for multi-stage cementing is often not very reliable.
A reduced density cement would thus be desirable, to increase the length of each section while keeping the mechanical properties of the set cement sufficient to ensure long-term isolation.
The present invention provides particularly light cementing formulations with good mechanical, impermeability, and adhesive properties.
For oil well cements, the technique most frequently used to reduce the density of a cement slurry consists of adding a larger quantity of water and extenders which are intended to prevent particles from separating out and/or to prevent the formation of free water on the slurry surface. Such a technique greatly reduces the compressive strength of the cement, increases its permeability and reduces the ability of the cement to adhere to supports. For those reasons, the technique cannot be used to produce densities less than of the order of 1300 kg/m3 and keep the geological layers isolated, as well as provide a sufficient casing strength.
A further routine technique for lightening a cement consists of formulating a slurry containing a surfactant, and introducing a gas such as air or nitrogen into the cement before it sets. The quantity of gas added is such that the required density is obtained. The quantity can be such that foamed cements are formed. A “foam quality” can be defined for such systems as the ratio of the volume of gas to the volume of foamed product, also the “swell” as the ratio of the increase in volume due to foaming to the volume of the foam. That technique is a little less powerful than the previous technique, as the density of gas is lower than that of water and thus less has to be added. However, the density is in practice limited to densities of over 1100 kg/m3 in oil industry applications even when starting from a slurry which has previously been lightened with water. Above a certain “foam quality”, the stability of the foam decreases very rapidly, the compressive strength of the set foam becomes too low and its permeability becomes too high which compromises its service life in hot aqueous media including ions which are aggressive towards the cement to a greater or lesser extent. In this regard, United States patent U.S. Pat. No. 5,696,059 should be consulted, as it discloses a foamed cement with a density of 1170 kg/m3, obtained with a foam quality in the range 30% to 35% and which, after 24 hours, has a compressive strength of only 4.2 MPa (607 psi) while the setting temperature is over 100° C. and the system comprises a micro-cement and silica.